List of documents filtered by Project Name, but want to include blank ones - essentially these are blank because they have not had an initial checkin.

?&FilterName=Project%5Fx0020%5FName&FilterMultiValue=iPads;Cathodic

Filters nicely bringing back to the 2 projects

aspx?&FilterField1=Projects&FilterValue1=

Will filter nicely as a single filter and only show blanks fields.

Need to incorporate the blank filter into the multi filter so users will also be able to see documents that have not been checked in. As there is a number of projects do not want to create seperate views for each.

I have to add: if you mean CAML <IsNull> elements, unfortunately this will not work with DFWP query string parameters, because <IsNull> clause only can be applied to field values :( Otherwise, i.e. if you mean XSLT filters, this will not do for large lists.
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Andrey MarkeevJan 23 '12 at 19:45